Considerable interest has been shown in photo-alignment methods for the fabrication of optical components including optical films. As just one example among many, U.S. Pat. No. 6,061,138 (Gibbon et al.) discloses a photo-alignment process using UV light for aligning a liquid crystal medium. Among the advantages of photo-alignment methods over traditional mechanical alignment methods, such as rubbing, is the capability to generate a high resolution pattern in the exposed photosensitive medium. Photo-alignment techniques can be used to form patterned alignment layers such as those used in an liquid crystal display (LCD) to create multi-domain liquid crystal structures and patterned optical films used as image cards, security cards, and the like. Depending upon the response characteristics of the photosensitive medium, patterns can be formed by changing various characteristics of the exposure energy, typically intensity or polarization, for example.
In photolithography, used, for example, in the manufacture of electronic circuits on silicon, a geometric pattern is imposed onto a photoresist wafer substrate by applying a mask to selectively block the exposure energy over specific areas. This same concept, using a static mask, can be applied to optical film fabrication, where the same regular pattern is used repeatedly as part of the fabrication process.
Driven in part by the high costs and schedule impact of mask fabrication and the difficulty of changing masks to correct for errors or to rework a fabrication process, the semiconductor industry has shown significant interest in “maskless” photolithography. Using this approach, as exemplified in U.S. Pat. No. 5,691,541 (Ceglio et al.) and U.S. Pat. No. 6,060,224 (Sweatt et al.), a digital micromirror device (DMD) is used to reflect a pattern onto the photoresist substrate. The DMD comprises an array of tiny mirrors, each separately addressable for reflecting light onto or away from the photoresist substrate. The exposure pattern can thus be formed on the DMD. Then, the exposure source is reflected from the DMD surface and focused onto the substrate for forming circuitry components. This approach appears to have merit where light intensity is used to form the desired pattern.
For many types of optical film, such as the film treated using the apparatus of U.S. Pat. No. 6,061,138, for example, the polarization state and direction of light, rather than light intensity, is used for conditioning alignment or other pattern formation in a photosensitive medium. Following the photolithography paradigm, masks have been employed in a number of devices used for exposure of optical film with polarized light, as is disclosed in U.S. Pat. No. 5,389,698 (Chigrinov et al.) It can be expected, however, that some of the same cost and deployment difficulties confronted when using masks in microlithography will also discourage the use of masks with optical film fabrication.
In an attempt to meet the need for an adaptive solution, U.S. Patent Application Publication No. 2002/0027624 A1 (Seiberle) discloses a transmissive optical component that acts as a mask, producing patterned, linearly polarized light. FIGS. 1A and 1B illustrate the basics of the process described for exposure of an optical film 40 comprising a photosensitive layer 20 and a substrate 10. A light source 1 emits a polarized light 91 having p-polarization. Transmissive mask 98, a transmissive LCD spatial light modulator, selectively modulates exposure beam 92 over each pixel area to provide either p-polarized light, as shown in FIG. 1A, or s-polarized light, as shown in FIG. 1B. As with the DMD solution for microlithography described above, the solution of U.S. Patent Application Publication No. 2002-0027624 A1 offers a mask solution that is dynamically changeable, eliminating the need to prepare and deploy separate masking components to support optical film fabrication.
When used for photo-alignment, photosensitive layer 20 is typically a thin photo-reactive alignment medium, typically linear photo-polymerization media (LPP), also known as photo-oriented polymer network (PPN). Photosensitive layer 20 is applied to substrate 10 and is then irradiated, typically using UV light, to provide a directional alignment bias.
There are a number of photo-alignment methods, based on different photoreaction processes. In general, a photo-alignment method may be one of three basic types:                (1) Isomerization, as disclosed in U.S. Pat. No. 4,974,941 (Gibbons et al.);        (2) Photo-dimerization, as disclosed in U.S. Pat. No. 5,602,661 (Schadt et al.); and        (3) Photo-dissociation, as taught in “Prospects of the photo-alignment technique for LCD fabrication” SID Digest 1997, pp. 311-314 (Iimura et al.).        
Once photosensitive layer 20 is aligned, a liquid crystal polymer (LCP) layer is applied over the LPP layer that has been treated to provide a preferred alignment orientation. As is well known in the photoaligment art, LCP materials include cross-linkable liquid crystalline monomers, oligomers, or pre-polymers having cross-linkable groups. Depending on the intended application, the cross-linkable liquid crystal material may exhibit a nematic, smectic, or cholesteric phase.
Although transmissive LCD spatial light modulators have been used successfully in a number of projection display apparatus, these devices have inherent disadvantages for high-energy exposure environments. As is well-known in the optical arts, transmissive LCDs are fabricated onto a clear substrate, so that circuit traces and componentry reduce the active area of each LCD cell that modulates a pixel. As a result, the effective aperture size for each pixel is a fraction of the area available on the transmissive LCD. This is a significant constraint on the amount of light available for each pixel, whether considered in terms of brightness, irradiance, or exposure energy. As a result, higher energy light sources must be used, with low levels of efficiency. Due to device geometry, there are also limitations to the resolution levels that can be achieved. Thus, while the transmissive LCD of U.S. Patent Application Publication No. 2002/0027624 has some capability for adaptive pattern forming on optical film, there are inherent limitations with respect to allowable intensity and resolution. Imaging at other than 1:1 magnification presents further difficulties, due to interference effects and focus restrictions.
Reflective LCD spatial light modulators have been used to modulate the polarization of incident light in imaging apparatus for display and projection. In these devices, light directed to the reflective LCD via a polarization beamsplitter is modulated by selectively altering the polarization state of individual pixels of incident light. The reflected light is sent back in the same direction as the incident light, but is transmitted through the polarization beamsplitter along its way to a display surface. For example, LCD spatial light modulators have been developed and employed for digital projection systems for image display, such as is disclosed in U.S. Pat. No. 5,325,137 (Konno et al.) and in miniaturized image display apparatus suitable for mounting within a helmet or supported by eyeglasses, as is disclosed in U.S. Pat. No. 5,808,800 (Handschy et al.) Improvements in reflective LCD components have improved the resolution of these devices. Because each pixel cell in the LCD array is reflective, aperture sizes are maximized, providing significant improvements in brightness or irradiance when compared with transmissive LCDs. However, disclosed uses of reflective LCD components are in image-forming for print and projection apparatus.
Thus, it can be seen that, while there have been some solutions presented for fabricating a patterned optical film by exposure of photosensitive materials using masks, there is room for improvement, particularly with respect to light efficiency and resolution.